Larkin, still officially general secretary of the ITGWU, clashed bitterly with William O'Brien, who had taken leadership of the ITGWU, the Irish Labour Party and the Irish Trades Union Congress.
During Larkin's absence at the 1924 Comintern congress (and possibly against his wishes), his brother Peter led their supporters out of the ITGWU, forming the WUI.
The new union quickly grew, gaining the allegiance of about two thirds of the Dublin membership of the ITGWU and of a smaller number of rural members.
Larkin and the WUI played a leading role in the unsuccessful campaign against the bill.
After Big Jim Larkin's 1947 death, his son James Larkin Jnr became general secretary, and continued to preside over a gradual expansion of the WUI, including amalgamations with a number of other unions.